340 



SEWAGE 



fin. 

 the 



interference, and should be independent of other 

 pipes and connection* with the sewers. Openings 

 at the levels of the streets have been largely used 

 for ventilators on the score of cheapness, though 

 they are generally a source of complaint at some 

 periods of the year as being a serious nuisance. 

 Sometimes the ventilating gratings of sewers in 

 street* are protected by means of charcoal air- 

 screens, as strongly urged by Dr Stenhouse ; and 

 when such screens have been adopted and are so 

 constructed as not to interfere with the free in- 

 gress and egress of air from the sewers, they have | 

 been found of great advantage, and have immensely 

 reduced the nuisance ami probable danger arising < 

 from an unprotected street sewer grating. 



To secure a sufficiency of fall tor the sewers in 

 order to make them self-cleansing it may be neces- 

 sary to divide a town into a number of sections ; 

 smaller sewers with rapid falls convey the sewage 

 witb rapidity to a number of different points, and 

 at these, points it may In- pumped away. Several 

 methods for the automatic pumping of the sewage 

 have been introduced. Mich as the hydraulic syst 

 the vacuum system of licrlier and Liernur, and 

 compressed air system of Shone. 



The disposal ofttwage is a most important point, 

 since crude sewage may no longer be discharged 

 into the fresh-water streams of the country. Sew- 

 age-irrigation has been very largely adopted as a 

 means of purifying the sewage, though the expected 

 returns from the manuring elements applied to the 

 land have been realised in only a very few instances. 

 The sewage-fanns of Berlin, however, have been 

 very successful, from engineering, sanitary, and 

 economic points of view, and waste-lands have been 

 made to yield large quantities of garden products. 

 But only in cases where it is not absolutely neces- 

 sary to purify the sewage at all times by its 

 application to the land can it be said to be remuner- 

 ative, owing to the difficulties which local author- 

 ities have in acquiring land for this purpose, and the 

 large sums of money to be paid by way of purchase, 

 often with a considerable contribution for conse- 

 quential damages arising from some supposed injury 

 to adjoining lands. Nor are the climatic conditions 

 in Britain favourable to the continual application 

 of liquid manures to land. Wherever, however, 

 sewage can be applied or not as required, as in 

 Craigentinny Meadows at Edinburgh, it has been 

 found to produce large and valuable crops of grass ' 

 well suited for the feeding of cattle. In the case 

 of ( 'roydon, where sewage-irrigation has been j 

 carried out more with a view to effect the purifica- 

 tion of the sewage, it has been found to have i 

 answered every purpose excepting that of making 

 a profit. Here the crude sewage, after having the 

 solids screened from it by means of a revolving 

 screen, actuated by the flow of the sewage, passes 

 on to tlie land and thence, after its purification, 

 into the river Wandle a river so small that the 

 How of sewage forms a very large percentage of the 

 total (low, and vet valuable as a trout stream. The 

 fact that the effluent sewage is passed into it with- 

 out injury to the fisheries speaks well for the 

 capability of a suitable soil to effect the purifica- 

 tion of sewage. 



Where Irrigation (q.v.) is adopted for the purpose 

 of purifying sewage, if the land has considerable 

 inclination, the irrigation is usually laid out on 

 the CAtchwork plan or with contour carriers one 

 above another which shed the sewage on the 

 pace below. If the land has a gentle fall, then 

 the sewage is Ix-st distributed over it on the pane 

 and gutter system gutters are cut down in the 

 direction of KM fall of the land at distances from , 

 half a chain to a chain apart, and from these the 

 sewage is thrown on to the intervening land by 

 of stops, which are removed from time to 



time. In the case of very flat land the ground in 

 laid out upon the bed system i.e. the sewage is 

 brought upon the ton of a sloping bed and falls 

 down to a gutter at the bottom of the slope of the 

 bed. In sewage-irrigation works, when purity of 

 effluent is desired, it will be found advisable to so 

 lay out the land as to be able to pick up the 

 effluent sewage which has passed over one area, 

 and to pass it a second, or even a third time, over 

 another plot of land, so as to ensure that no liquid 

 has passed away without being purified. 



Another method of purifying sewage is by inter- 

 mittent filtration through land i.e. the land is 

 laid out in plots to form a filter, which must be 

 effectually drained, and the sewage, l>eing placed 

 upon a particular plot, is allowed to filter through 

 the land to the drains In-low . The filtration area 

 is so arranged that the plots are used intermit- 

 tently or in succession, and in this way a limited 

 area of land may be made to purify a very consider- 

 able volume of sewage ; the more porous the land, 

 the more sewage it will purify. By this inter- 

 mittent action a considerable degree of purity is 

 secured in the effluent sewage, and a suitable crop 

 may be grown upon the surface of such filters. 

 Intermittent filtration areas are in common use 

 in connection with most irrigation farms so as to 

 avoid as far as possible the application of the 

 sewage to large areas of land in the winter and 

 at other times when the land is under crop not 

 suited for the application of large volumes of 

 sewage. At the works for the Croydon rural 

 district at Merton, and for the Kingston rural 

 district at Esher in Surrey, the whole of the 

 sewage is treated by intermittent filtration, and 

 these works are typical representatives of this 

 system. In the former case the sewage is applied 

 in its raw state after simple subsidence to remove 

 the solid matters in suspension ; and in the latter 

 case the sewage is chemically treated before its 

 application to the land. 



It has been found that the purification of sewage, 

 whether by irrigation or intermittent filtration 

 through land, is entirely due to a small organism 

 discovered by Messrs Schilling and Muntz in 

 connection with the Paris Sewage Farm. This 

 microbe has the power of converting nitrogenous 

 matter into nitric acid ; and investigations made 

 by Professor Warrington at the laboratory of Sir 

 John Bennett Lawes and Dr (;ilbert at Kotfiamsted 

 show that it is mostly to be found in surface-soils 

 (see NITRIFICATION). It is not found at any depth 

 below the surface. MCSMS Schloesing and 'Muntz 

 showed experimentally that if the soil containing 

 the nitrifying organism was chloroformed the organ- 

 isms were rendered inactive, and in this state 

 sewage could pass through the soil without purifica- 

 tion, out nitrification and purification was resumed 

 when the organism woke up. Since this dis- 

 covery it has been shown that artificial filters may 

 be built of suitable soils and other ]>orous materials 

 which allow the ready admittance of atmospheric 

 air, so that large volumes of sewage may be dealt 

 with upon limited areas. At the worn of the 

 Kriern Barnet Local Board at New Soutligate. the 

 sewage of upwards of fdlOO people per acre lias from 

 Is.Vi till 1892 been effectually filtered and purified 

 after chemical treatment by being pas-cd through 

 artificial filters; mid during the whole of the time 

 these filters have been in operation they have not 

 had a particle of material removed from their 

 surface. Experience shows, however, that these 

 filters can be put out of order by paralysing the 

 action of the nitrifying organism by giving them 

 an excessive dose of chemicals in the sewage. 



Experiment* made by the State Board of Health 

 of Massachusetts on intermittent filtration tend to 

 show that coarse sand when used intermittently ia 



